There are a variety of causes – usually locales not being generated properly or incorrect environment variables.

Lately I’ve been getting these a lot of these, but only some of the time. After some messing around I eventually discovered the problem wasn’t on the server itself. The problem only occurs when I’m logged in over SSH from a Mac. The default sshd on Debian copies LANG and LC_* variables from the client. The LC_CTYPE variable was set on my Mac, apparently by default, to “UTF-8”. While this seems to work on MacOS X, it’s not a valid value for Linux.

The easiest solution was just to add the following to my ~/.profile on the Mac: